With the development in portable devices such as personal computers and cell phones, demand is growing for batteries as power sources for such devices.
In batteries for such application, an electrolyte comprising a liquid such as organic solvent has been used as a medium for transferring ions. For this reason, there is a possibility that problems such as leakage of electrolyte from the battery might occur.
In order to solve the above problems, development is under way to produce an all solid state battery using, instead of a liquid electrolyte, a solid electrolyte. An all solid state lithium secondary battery, in particular, is vigorously being studied in many fields as a battery capable of providing a high energy density. This is because, for example, the use of Li metal in a negative electrode active material yields a high electromotive force since Li has a low atomic weight and the greatest ionization tendency, and therefore it is the most electrochemically base metal.
As the solid electrolyte used for the all solid state lithium secondary battery, for example, lithium halide, lithium nitride, lithium oxoacid salt and derivatives thereof are well known. Particularly, lithium phosphorus oxynitride (LixPOyNz, where x=2.8, 3z+2y=7.8) obtained by introducing nitrogen (N) into lithium orthophosphate (Li3PO4) is reported to have a very high lithium ion conductivity of (1 to 2)×10−6 S/cm although it is an oxide based material (see U.S. Pat. No. 5,597,660).
When the lithium phosphorus oxynitride is exposed to a wet atmosphere, however, phosphorus atoms (P) constituting lithium phosphorus oxynitride reacts with water molecules present in the wet atmosphere, during which the phosphorus atoms are reduced to a lower oxidation state from an oxidation state of +5. Thereby, lithium phosphorus oxynitride is decomposed, which significantly reduces the ion conductivity thereof.
When the ion conductivity is reduced as described above, internal impedance will increase in an all solid state battery using a solid electrolyte composed of lithium phosphorus oxynitride. For this reason, its charge/discharge rate characteristics will be significantly impaired.
Further, the decrease in ion conductivity in a wet atmosphere is also observed in lithium phosphate. Accordingly, such decrease in ion conductivity is the underlying problem in materials for solid electrolyte having lithium phosphate in the framework structure.
In view of the above, an object of the present invention is to provide a solid electrolyte capable of suppressing the decrease in ion conductivity even in a wet atmosphere, and an all solid state battery using the solid electrolyte.